496

32 Microbiology of Biogas Production from Food Waste: Current Status, Challenges, and Future Needs

Anaerobic digestion with animal feedstock generally results in 60% methane and

40% carbon dioxide. As acetate and hydrogen form substrate for various other chem-

ical reactions, the biogas yield in an anaerobic digestion is influenced by digester

conditions, microbial consortia, and the substrate [3].

The decomposition in the digester halts when the concentration of methanogenic

Archaea drops below the threshold value of the total consortia population associated

with low biogas and accumulation of volatile fatty acids (VFA), respectively. Rod-like

or coccoid hydrogenotrophic methanogens, Methanosarcina species are favored in

thermophilic condition. Methanogenic archaea are weakest in food wastes, distillery

wastes, etc. [3, 5].

About 70–75% methane is generated from acetate produced from the feedstock

and the rest from hydrogen and CO2. High hydrogen concentration can inhibit ace-

togenesis resulting in accumulation of fatty acids [6].

32.5

Pretreatment

Generally, pretreatment is given to feedstock rich in lignocellulose or keratinase as

it causes complex floc structures of microbial biomass. Hence, degradation of such

waste can be achieved by physical (heat, pressure), chemical (acid, base, ozonation),

and biological (coculturing, enzyme addition) pretreatments. Such pretreatments

facilitate biomass porosity-enhanced surface area for microbial degradation. But,

certain pretreatments can result in the production of inhibitors such as furfural,

vanillin, and other phenolic compounds [5].

32.6

Variations in Anaerobic Digestion

Hydrolysis of plant-based feedstock is delayed owing to the organization of com-

plex cellulosic biomass. Recalcitrant lignin is known to be detached by alkali

treatment. Beet silage-fed digester in mesophilic phase and continuous operation

revealed bacterial consortia of Proteobacteria, Bacteroidetes, Actinobacteria, and

Chloroflexi in different percentages with increasing days of operation. The same

reactor under thermophilic conditions had Clostridia, Firmicutes, Bacteroidetes,

and Proteobacteria in varying concentration with increasing days of operation.

A continuously stirred tank reactor (CSTR) fed with straw was dominated by

Clostridia, Bacteroidetes, Acidobacteria, Deltaproteobacteria, while the digester with

animal waste feedstock recorded prevalence of other species, like Ruminococcus

and Cellulomonas.

Clostridium thermocellum

and

Clostridium stercorarium

were

the

prime

microflora responsible for digestion of cellulose and hemicellulose, respectively, in

maize silage in thermophilic phase resulting in ethanol, lactate, acetate, butyrate,

and other short-chain products in addition to CO2 and H2 gas. Hydrogenotrophic

methanogen, Methanothermobacter thermautotrophicus can be co-cultured with

Tepidanaerobacter which hydrolysis lactate. The predominant microflora in grass